Phytochrome―"Pr" absorbs red light of wavelength.

Explanation

Phytochrome exists in two forms i.e. P 660 and P 730. P 660, a quiescent form, absorbs red light at a wavelength of 660 nm and is converted to active P 730, P 730 absorbs far-red light at 730 nm and is converted to P 660. In nature, the P 660 to P 730 conversion takes place in daylight and the P 730 to P 660 conversion occurs in the dark. Thus during the day, a plant has P 730 phytochromes while during the night it contains more phytochromes in the form of P 660. The presence of either form provides the plants with a means of detecting whether it is in a light or dark environment. The rate at which P 730 is converted to P 660 provides the plant with a "clock" for measuring the duration of darkness.